Surfactants are essential components of everyday products such as household and industrial cleaners, agricultural products, personal care products, laundry detergents, oilfield chemicals, specialty foams, and many others.
Modern laundry detergents perform well in removing many kinds of soils from fabrics when warm or hot water is used for the wash cycle. Warmer temperatures soften or melt even greasy soils, which helps the surfactant assist in removing the soil from the fabric. Hot or warm water is not always desirable for washing, however. Warm or hot water tends to fade colors and may accelerate deterioration of the fabric. Moreover, the energy costs of heating water for laundry make cold-water washing more economically desirable and more environmentally sustainable. In many parts of the world, only cold water is available for laundering articles.
Of course, laundry detergents have now been developed that are designed to perform well in hot, warm, or cold water. One popular cold-water detergent utilizes a combination of a nonionic surfactant (a fatty alcohol ethoxylate) and two anionic surfactants (a linear alkylbenzene sulfonate and a fatty alcohol ethoxylate sulfate) among other conventional components. Commercially available cold-water detergents tend to perform well on many common kinds of stains, but they have difficulty removing greasy dirt, particularly bacon grease, beef tallow, butter, cooked beef fat, and the like. These soils are often deposited as liquids but quickly solidify and adhere tenaciously to textile fibers. Particularly in a cold-water wash cycle, the surfactant is often overmatched in the challenge to wet, liquefy, and remove these greasy, hardened soils.
Most surfactants used in laundry detergents have a polar head and a nonpolar tail. The polar group (sulfate, sulfonate, amine oxide, etc.) is usually located at one end of the chain. Branching is sometimes introduced to improve the solubility of the surfactant in cold water, especially for surfactants with higher chain lengths (C14 to C30), although there is little evidence that branching improves cold-water cleaning performance. Moreover, even the branched surfactants keep the polar group at the chain terminus (see, e.g., U.S. Pat. Nos. 6,020,303; 6,060,443; 6,153,577; and 6,320,080).
Secondary alkyl sulfate (SAS) surfactants are well known and have been used in laundry detergents. Typically, these materials have sulfate groups that are randomly distributed along the hydrocarbyl backbone. The random structure results from addition of sulfuric acid across the carbon-carbon double bond in internal olefin mixtures, accompanied by double bond isomerization under the highly acidic conditions.
Recognizing the solubility limitations of conventional secondary alkyl sulfates in cold water, U.S. Pat. No. 5,478,500 teaches to combine them with optimum levels of an amine oxide surfactant and a linear alkylbenzene sulfonate.
Secondary alkyl sulfates have been produced in which the sulfate group resides at the 2- or 3-position of the alkyl chain (see, e.g., WO 95/16016, EP 0693549, and U.S. Pat. Nos. 5,478,500 and 6,017,873). These are used to produce agglomerated high-density detergent compositions that include linear alkylbenzene sulfonates, fatty alcohol sulfates, and fatty alcohol ether sulfates. Similarly, U.S. Pat. No. 5,389,277 describes secondary alkyl sulfate-containing powdered laundry detergents in which the alkyl chain is preferably C12-C18 and the sulfate group is preferably at the 2-position.
Longer-chain (C14-C30) surfactants have been produced in which the polar group resides at a central carbon on the chain, but such compositions have not been evaluated for use in cold-water laundry detergents. For example, U.S. Pat. No. 8,334,323 teaches alkylene oxide-capped secondary alcohol alkoxylates as surfactants. In a few examples, the original —OH group from the alcohol is located on a central carbon of the alkyl chain, notably 8-hexadecanol and 6-tetradecanol. As another example, sodium 9-octadecyl sulfonate has been synthesized and taught as a surfactant for use in enhanced oil recovery (see J. Disp. Sci. Tech. 6 (1985) 223 and SPEJ 23 (1983) 913). Sodium 8-hexadecyl sulfonate has been reported for use in powder dishwashing detergents (see, e.g., JP 0215698).
Numerous investigators have studied a series of secondary alcohol sulfates in which the position of the sulfate group is systematically moved along the alkyl chain to understand its impact on various surfactant properties. For example, Evans (J. Chem. Soc. (1956) 579) prepared a series of secondary alcohol sulfates, including sodium sulfates of 7-tridecanol, 8-pentadecanol, 8-hexadecanol, 9-septadecanol, 10-nonadecanol and 15-nonacosanol (C29), and measured critical micelle concentrations and other properties. More recently, Xue-Gong Lei et al. (J. Chem. Soc., Chem. Commun. (1990) 711) evaluated long-chain (C21+) alcohol sulfates with mid-chain branching as part of a membrane modeling study.
Dreger et al. (Ind. Eng. Chem. 36 (1944) 610) prepared secondary alcohol sulfates having 11 to 19 carbons. Some of these were “sym-sec-alcohol sulfates” in which the sulfate group was bonded to a central carbon (e.g., sodium 7-tridecyl sulfate or sodium 8-pentadecyl sulfate). Detergency of these compositions was evaluated in hot (43° C.) water. The authors concluded that “when other factors are the same, the nearer the polar group is to the end of a straight-chain alcohol sulfate, the better the detergency.” Cold-water performance was not evaluated.
Similarly, Finger et al. (J. Am. Oil Chem. Soc. 44 (1967) 525) studied the effect of alcohol structure and molecular weight on properties of the corresponding sulfates and ethoxyate sulfates. The authors included sodium 7-tridecyl sulfate and sodium 7-pentadecyl sulfate in their study. They concluded that moving the polar group away from the terminal position generally decreases cotton detergency and foam performance.
Mid-chain surfactants having functional groups other than sulfates have been described. U.S. Pat. Appl. Publ. No. 2007/0111924, for instance, teaches liquid laundry detergents comprising a sulfate or sulfonate component and a mid-chain amine oxide. Mid-chain sulfonates, sometimes referred to as “double tailed” sulfonates, are also known (see, e.g., R. Granet et al., Colloids Surf. 33 (1988) 321; 49 (1990) 199); the performance of these materials in laundry applications has not been reported.
Internal olefin sulfonates are well known. Although they are useful for enhanced oil recovery (see, e.g., U.S. Pat. Appl. No. 2010/0282467), they have also been suggested for use in detergent compositions, including laundry detergents (see U.S. Pat. No. 5,078,916). These are prepared by sulfonating mixtures of internal olefins. Commercially available internal olefins, including the Neodene® products of Shell, are generated by isomerizing alpha olefins in the presence of a catalyst that also scatters the location of the carbon-carbon double bond. Consequently, sulfonates made from the internal olefins (including the commercial Enordet® products from Shell) do not have a well-defined location for the polar group.
Surfactants in which the polar group is separated from the principal alkyl chain by an alkylene bridge are known. Some methylene-bridged surfactants of this type are derived from “Guerbet” alcohols. Guerbet alcohols can be made by dimerizing linear or branched aliphatic alcohols using a basic catalyst using chemistry first discovered in the 19th century. The alcohols, which have a —CH2— bridge to the hydroxyl group near the center of the alkyl chain, can be converted to alkoxylates, sulfates, and ether sulfates (see, e.g., Varadaraj et al., J. Phys. Chem. 95 (1991), 1671, 1677, 1679, and 1682). The Guerbet derivatives have not apparently been shown to have any particular advantage for cold-water cleaning.
Surprisingly few references describe surfactants that demonstrate improved cleaning using cold water (i.e., less than 30° C.). U.S. Pat. No. 6,222,077 teaches dimerized alcohol compositions and biodegradable surfactants made from them having cold water detergency. A few examples are provided to show improved cold water detergency on an oily (multisebum) soil when compared with a sulfated Neodol® C14-C15 alcohol. Made by dimerizing internal or alpha olefins (preferably internal olefins) in multiple stages followed by hydroformylation, these surfactants are difficult to characterize. As shown in Examples 1-3 of Table 1 of the '077 patent, NMR characterization shows that a single dimerized alcohol product typically has multiple components and a wide distribution of branch types (methyl, ethyl, propyl, butyl, and higher) and various attachment points on the chain for the branches. A high degree of methyl branching (14-20%) and ethyl branching (13-16%) is also evident.
PCT Int. Appl. No. WO 01/14507 describes laundry detergents that combine a C16 Guerbet alcohol sulfate and an alcohol ethoxylate. Compared with similar fully formulated detergents that utilize a linear C16 alcohol sulfate, the detergent containing the Guerbet alcohol sulfate provides better cleaning in hot (60° C.) or warm (40° C.) water. Laundering with cold (<30° C.) water is not disclosed or suggested.
PCT Int. Appl. No. WO 2013/181083 teaches laundry detergent compositions made by dimerizing even-numbered alpha-olefins to produce vinylidenes, hydroformylation of the vinylidenes to give alcohols mixtures, and sulfation of the alcohols. Hydroformylation is performed in a manner effective to provide alcohol mixtures in which methyl-branched products predominate. According to the inventors, methyl branching on even-numbered carbons on the alkyl chain is believed to contribute to rapid biodegradation in sulfate surfactants made from the alcohols. When compared with similar sulfates having random branching on the chain, those with branching on even-numbered carbons had similar cleaning ability at 20° C. but improved biodegradability.
Improved detergents are always in need, especially laundry detergents that perform well in cold water. Of particular interest are detergents that can tackle greasy dirt such as bacon grease or beef tallow, because these stains solidify and adhere strongly to common textile fibers. Ideally, the kind of cleaning performance on greasy dirt that consumers are used to enjoying when using hot water could be realized even with cold water.